Detector for indicating a fire or detector malfunction

ABSTRACT

A fire detector provided with a warning system, a reference element and an analysis element whose impedance varies in the presence of a rise of temperature and/or radiation and/or combustion gas, said reference and analysis elements being connected in series or in parallel and constituting two adjacent arms of a Wheatstone bridge assembly. The other two arms are constituted by resistors if necessary variable while the measuring diagonal or bridge, joins, either the common connecting points of the resistors and said elements, or respectively the ends of the detector elements. This device is distinguished in that the two ends of the bridge (X and Y) are connected respectively to the two inputs of two voltage comparators (A 1  and A 2 ), mounted in parallel and in reverse, each connected to a warning system (S 1 , S 2 ) so that unbalance of the bridge creates a signal at the output of one or other comparator according to the direction of unbalance and actuates the corresponding warning system to indicate, respectively, a fire or system malfunction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a fire detector.

2. Description of the Prior Art

Numerous types of fire detectors or alarms exist, such as temperaturerise detectors generally comprising thermistors, detectors of radiationemitted by a flame and including photoelectric cells or a brushdischarge tube and smoke or combustion gas detectors. The latter arebased on optical phenomena or even include at least one ionizationchamber traversed by a current which varies according to the gasespassing through it.

All these devices are connected electrically to a warning or alarmsystem controlled by a device which establishes the difference between ameasured value and a reference value corresponding to a normal ortolerable level. These devices generally require a considerableconsumption of current which is particularly heavy considering that theyoperate continuously. In addition, they are sensitive to disturbingexternal phenomena and use, for the ionization detectors, considerableradioactive sources.

It is an object of the invention to provide a particularly reliable andlow comsumption electronic system for fire detection purposes. This isbased on a comparative measurement between a measured value and areference value.

It is another object of the invention to provide a warning system in thecase of faulty operation of the fire detection device.

Other objects and advantages of the fire detector system according tothe invention will be understood on reading the description whichfollows.

GENERAL DESCRIPTION OF THE INVENTION

The fire detection system according to the invention is designed to beadapted to numerous types of electrical detectors which include awarning or alarm system, a reference element and an analysis elementwhose impedance varies in the presence of a rise of temperature and/orof radiation and/or combustion gas, said reference and analysis elementsbeing connected in series or in parallel and constituting two adjacentarms of a Wheatstone bridge circuit whose two other arms are constitutedby resistors, if necessary variable, whilst the measuring diagonal orbridge, joins, either the common connecting points of the resistors andsaid elements, or respectively the ends of the detector elements.

The device according to the invention is remarkable in that the two endsof the bridge are connected respectively to two inputs of two voltagecomparators, mounted in parallel and in reverse, each connected to awarning system such that unbalance of the bridge creates a signal at theoutput of one or other comparator according to the direction ofunbalance and actuates the corresponding warning system.

Advantageously, in this case hysteresis is created between the twotriggering actions by means, for example, of a voltage divider bridgearranged at one of the inputs of one of the comparators.

Experience shows that the comparators are advantageously constituted bylinear amplifiers whose gain is programmable.

It has been found, moreover, after numerous studies and tests that theamplifiers which permit good operation are integrated operationalamplifiers known by the connotations LM 4250 or μA 776.

It is clear that this arrangement is adaptable to numerous types ofdetectors and the reference and analysis elements may be constituted bybrush discharge tubes sensitive to radiation or thermistors sensitive torise in temperature or ionization chambers sensitive to combustiongases.

Ionization chambers may, in addition, include electrodes at reducedvoltage one of which is provided with a radioactive source or electrodessubject to high voltage with or without a radio-element.

It is also possible for one at least of the comparators to be precededby at least one impedance adaptor device such as a field effecttransistor.

In the case of a detector whose reference and analysis elements areconstituted by two radioactivity ionization chambers provided ifnecessary with a common electrode, it has appeared to be particularlyimportant for the height (i.e., electrode spacing) of the analysis andreference chambers to be respectively 12.5 mm and 7.5 mm.

A device according to the invention possesses high sensitivity due toits measurement by comparsion, and enables a fire warning system or afault warning system to be actuated according to the direction ofunbalance of the bridge. In its application to ionization detectors, theinvention enables the use of weak radioactive sources, which representsa considerable advantage in view of the strict regulations to which suchsources are subject and the risk of contamination which exists withhighly radioactive sources.

To make even weaker the radioactive sources used in the detectors it hasbeen conceived to increase even more the voltage applied to thechambers.

However, it has appeared contrary to the ideas received that thisvoltage had to be limited. Study of the ionization phenomenon shows thatthe graph of the ionization current as function of the voltage appliedto the system, has several zones.

In the first zone, so-called recombination zone, certain of the ionscontained in the chamber recombine together. This zone has hitherto beenvery little used in contrast with the other zones such as the ionizationchamber zone, the proportional counter zone or the Geiger-Muller zone.The inventor has discovered that this recombination zone which hencecorresponds to a poor ionization chamber was partivularly sensitive tosmoke or combustion gases. Consequently, according to one embodiment ofthe invention in which at least the analysis element is constituted by aradioactivity ionization chamber, the voltage applied to the terminalsof this analysis chamber is situated in this so-called recombinationzone of its graph of the ionization current as a function of the appliedvoltage.

In addition, it has been found particularly advantageous if thereference element be constituted by the internal resistance of a fieldeffect transistor arranged in the bridge circuit. In fact, this veryconsiderable internal resistance varies as a function of the externalparameters, other than the combustion gases, substantially in the samemanner as the analysis chamber and hence constitutes a good referenceelement.

In this case, it is advantageous for the source of the field effecttransistor to be connected to ground through a variable resistor whichenables the internal resistance of the field effect transistor to bevaried.

It is then possible, by means of this variable resistance to adjust thevoltage applied to the analysis chamber and in the case of parallelmounting to adjust the balance of the bridge.

To economize in energy, according to one embodiment, the detector isprovided with a low voltage general supply source and with a highervoltage generator for the analysis and reference elements controlled involtage by a regulator designed and arranged so as to deliver voltage tosaid generator only when the voltage of the latter remains less than areference voltage.

In this case the regulator is advantageously constituted by a comparatorof which one of the inputs is connected to a reference voltage sourceand the other input to a point in the circuit situated downstream of thegenerator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features will appearmore clearly from reading the description which follows with referenceto the accompanying drawing, in which:

FIG. 1 shows diagrammatically, a first embodiment of a detectoraccording to the invention, provided with two ionization chambers;

FIG. 2 shows a series arrangement similar to that of FIG. 1 in which thereference element is constituted by the internal resistance of a fieldeffect transistor;

FIG. 3 shows an embodiment similar to that of FIG. 2, but with anarrangement in parallel, and

FIG. 4 shows an embodiment with a parallel arrangement and provided witha voltage generator and a regulator.

DESCRIPTION OF PREFERRED EMBODIMENTS

Ionization detectors are known and include at least one chamber in whicha radioactive material is placed intended to ionize said chamber. Whensmoke or combustion gases enter the chamber, the ionization state ismodified so that the number of mobile charges diminishes, which amountsto an increase in the internal impedance of the chamber.

In the example shown in FIG. 1, the device includes two ionizationchambers 1 and 2 respectively for analysis and reference connected inseries. Each of the latter is provided with a radioactive material 3 and4. The chambers are arranged so that the positive electrode 5 of theanalysis chamber 1 and the negative electrode 6 of the reference chamberare connected to the common point B whilst a current source is connectedbetween the points A and C, that is to say between the positive andnegative electrodes of the respective chambers 2 and 1. (Source of 18 to30 V, for example).

The common point B is connected electrically to the input of aquadripole Q₁ itself followed by a quadripole Q₂. The quadripoles Q₁ andQ₂ are four-terminal impedance adaptors (e.g., field effect transistors)designed to lower the high output impedance of the chambers. It isobviously possible to use only a single impedance adaptor quadripole,such as a field effect transistor, for example, or any other equivalentcomponent.

Three resistors in series R₁, R₂ and R₃ are connected in parallelbetween the points A and C, the resistor R₂ being arranged as apotentiometer. The output X of the quadripole Q₂ and a variable point Yon the resistor R₂ are respectively connected to the negative andpositive inputs of an operational amplifier A₁. The output X is, inaddition, connected through a resistor R₄ to the positive input of anoperational amplifier A₂, whilst the point Y is connected electricallyto the negative input of the latter. The point Y is therefore connectedto the positive and negative inputs respectively of the amplifiers A₁and A₂, through, in the example shown, two input resistors R₆ and R₇.

The operational amplifiers A₁ and A₂ used as a comparator areadvantageously programmable by means of the resistors R_(B) and R_(B').The resistor R₄ arranged between the point X and the input of theamplifier A₂ constitutes one of the resistors of a divider bridge R₄,and R₅ whose middle point Z is directly connected to the above-mentionedinput.

It is clear that the chambers 1 and 2 constitute two adjacent arms of aWheatstone bridge, the two other arms being formed by the resistors R₁,R₃ and the variable resistor R₂ arranged between the latter whilst themeasuring diagonal or bridge is comprised between the points X and Y.

The amplifiers A₁ and A₂ are respectively connected to warning systemsS₁ and S₂ including, notably, quadripoles Q₃, Q₄ and resistors or relaycoils 7 and 8 designed to actuate any suitable light means and/or otherrelays.

When the voltage at X is less than the input voltage of the amplifierA₁, the latter trips and a signal triggers the warning system S₁. On theother hand, when the voltage Z is higher than the input voltage of theamplifier A₂, the latter trips and actuates the system S₂.

FIG. 1 shows two chambers 1 and 2 diagrammatically. In practice, thelatter are arranged in a box, possibly a common box, the analysischamber 1 being open and the reference chamber 2 completely or partlyclosed.

The operation of the system is easy to understand. The potentiometer R₂is adjusted so that the bridge is balanced. If external phenomena otherthan smoke emmisions modify the impedances of the two chambers, thebridge is not thrown out of balance and no warning system is actuated.On the other hand, if combustion gases enter the analysis chamber, thecurrent which passes through it is modified and the voltage at the pointZ becomes higher than the input voltage of the amplifier A₂, whichtriggers the fire warning system S₂. On the other hand, if the unbalanceof the bridge takes place in the other direction, that is to say if thevoltage at the point X becomes less than the input voltage of theamplifier A₁, it is the fault or failure warning system S₁ which istriggered. This triggering takes place, for example, following adeterioration of the impedance adaptor quadripoles Q₁, Q₂ or a breakageof the resistor arms or again, following poor adjustment of thepotentiometer R₂, etc.

The divider bridge R₄, R₅ enables a triggering threshold to be set byshifting the balance point of the bridge, so as to produce hysteresisbetween the triggering of fire and fault warnings respectively. Thisarrangement enables the sudden passage from one state to the other to beavoided.

In the device described, it has appeared to be particularly advantageousto use as the electronic components for the amplifers A₁ and A₂, theabove-mentioned operational amplifiers. In the same way the impedanceadaptors may be provided in the form of a single field effecttransistor, for example of the 2N 4416 type. In addition, the height ofeach of the chambers is important and it has already been stated thatthe analysis and reference chambers 1 and 2 advantageously have heightsof 12.5 mm and 7.5 mm.

It is clear that the series arrangement of the chambers 1 and 2 is notobligatory and it is possible to conceive an arrangement in parallel onthe supply, the chambers 1 and 2 constituting nonetheless two adjacentarms of the bridge arrangement as is shown in FIG. 4, for example, whichwill be discussed below.

FIGS. 2 and 3 show two very important embodiments of devices accordingto the invention. They show diagrammatically a series arrangement (FIG.2) similar to that of FIG. 1 and a parallel arrangement (FIG. 3) similarto that of FIG. 4. However, here the reference element (chamber 2 ofFIGS. 1 and 4) is replaced by the internal resistance Ri of a fieldeffect transistor TEC, the resistance Ri being shown in a dashed linesince it is an imaginary resistance.

FIG. 2 takes up the series arrangement of FIG. 1 and the resistors R₁,R₂ and R₃ are again to be found as well as the ends of the bridge X andY. This arrangement is particularly advantageous, since it enables theuse of a very weak radioactive source of the order, for example, of 0.05μC, that is to say, 1850 disintegrations per second. In fact, this lowradioactivity results in a high impedance of the analysis chamber 1,compensated by the internal resistance Ri of high value. In addition, ifthe bridge is adjustable by means of the variable resistor R₂, it ispossible to adjust the balance point by means of a variable loadresistor R_(s) arranged at the source of the TEC transistor whichenables the internal resistance Ri to be modified. This adjustment thuspermits the use of a wide range of radioactive sources.

FIG. 3 shows also a bridge arrangement with an analysis chamber 1, areference element constituted by the internal resistance Ri of the TECtransistor and resistors R_(A) and R_(R) constituting the two other armsof the bridge. However, to adjust the balance of the bridge, instead ofhaving a variable resistor R_(R), it is advantageous to provide as inFIG. 2, a variable load resistor R_(s) at the source of the TEC whichpermits the resistance Ri to be varied.

In addition, as has been stated, the voltage applied to the terminals ofthe analysis chamber is situated in the recombination zone of thelatter.

FIG. 4 shows an advantageous embodiment with two ionization chambers,respectively for analysis and reference 1 and 2, as in FIG. 1, butconnected in parallel. In this embodiment, the point Y is adjustable tothe resistance R_(R) of the reference chamber (similar to that of FIG.3) whilst the resistance R_(A) of the analysis chamber of FIG. 3 isconstituted by two resistors R₈ and R₉. This embodiment includes, asthat of FIG. 1, comparators A₁ and A₂ and warning systems showndiagrammatically at S₁ and S₂.

However, this embodiment is provided with a fairly low voltage supply,shown by small triangles, whilst chambers 1 and 2 are supplied by ahigher voltage generator (or the order, for example, of 15 to 30 volts)which includes in the example shown, an oscillator and a highpermeability transformer Tf, for example μ>1000 gauss, provided with twoprimary windings in series and a secondary winding. The oscillations aremaintained by a transistor T_(R1) controlled in voltage by a regulatorincluding an operational amplifier A₃ mounted as a comparator and atransistor T_(R2). The positive or non-inverting input of the amplifierA₃ is connected to a divider bridge constituted by resistors R₁₀, R₁₁,R₁₂ and the Zener diode Z which delivers a reference voltage, whilst thenegative or inverting input of said amplifier is connected to thecircuit between the resistors R₈ and R₉ which constitute a dividerbridge with the analysis chamber 1. With this arrangement, the amplifierA₃ only delivers voltage to the oscillator when the voltage thus tappedhas not reached the reference voltage applied to its positive input, sothat the secondary voltage at the oscillator is controlled, whilst itsconsumption is dependent on the secondary utilization consumption, whichis particularly low in this embodiment.

It is obvious that the embodiments described may be subject to a largenumber of modifications or be combined without departing from the scopeof the invention. In addition, the analysis elements described areconstituted by radioactivity ionization chambers and it is clear thatthe latter could be replaced by electrodes subjected to high voltage bymeans of a generator of the type described with reference to FIG. 4 orby other heat or radiation detector means. Finally, the embodiments ofFIGS. 1, 2 and 3 may obviously utilize a generator and a regulator ofthe type described with reference to FIG. 4.

I claim:
 1. A detector for a predetermined condition such as a rise intemperature or the presence of radiation or combustion gas comprising areference element and an analysis element whose impedance varies as afunction of said predetermined condition connected together as two armsof a Wheatstone bridge whose other two arms are constituted byresistors, a measuring diagonal of the bridge being connected to a pairof voltage comparators for respectively triggering warning systems, eachof the comparators having input terminals of opposite polarity and anoutput terminal that produces a control signal when the voltage on thepositive input terminal exceeds the voltage on the negative inputterminal, the improvement comprising: connecting one end of themeasuring diagonal to respective input terminals of opposite polarity ofthe comparators, connecting the other end of the measuring diagonal tothe other input terminal of a first of the comparators and to the otherinput terminal of a second of the comparators whereby control signalsproduced by the first and second comparators, in response to unbalancingof the bridge, trigger respective warning systems that indicate theexistence of a fault and the existence of said predetermined condition.2. Detector according to claim 1, includng hysteresis means to create ahysteresis between triggering of the two warning systems.
 3. Detectoraccording to claim 2, wherein said hysteresis means is a voltage dividerarranged at the other input of the one of the comparators.
 4. A detectoraccording to claim 2 wherein said hysteresis means is a resistorinterposed between said other end of the measuring diagonal and saidother input terminal of said second comparator.
 5. Detector according toclaim 1, wherein the comparators are constituted by linear amplifierswhose gain is programmable.
 6. Detector according to claim 5, whereinthe amplifiers are integrated operational amplifiers known by thereferences LM 4250 or μ A776.
 7. Detector according to claim 1, whereinat least one of the comparators is preceded by at least one impedanceadapter device (Q₁, Q₂) comprising a field effect transistor. 8.Detector according to claim 1 whose reference and analysis elements areconstituted by two radioactivity ionization chambers provided with acommon electrode and two other electrodes, wherein the space between thecommon electrode and the first of the other electrodes of the analysischamber is substantially 12.5 mm and the space between the commonelectrode and the second of the other electrodes of the referencechamber is substantially 7.5 mm.
 9. Detector according to claim 1 inwhich at least the analysis element is constituted by a radioactivityionization chamber, wherein the voltage applied to the terminals of thisanalysis chamber is situated in the so-called recombination zone of theionization current versus applied voltage curve.
 10. Detector accordingto claim 1, whose analysis element is constituted by a radioactivityionization chamber and wherein the reference element is constituted bythe internal resistance (Ri) of a field effect transistor (TEC) arrangedin the bridge assembly.
 11. Detector according to claim 10, wherein thesource of the field effect transistor (TEC) is connected to groundthrough a variable resistance (Rs) which enables the internal resistanceof the field effect transistor to be varied.
 12. Detector according toclaim 1, provided with a general supply low voltage source and with agenerator of higher voltage for the analysis and reference elementscontrolled in voltage by a regulator designed and arranged so as todeliver voltage to said generator only when the voltage of saidgenerator remains less than a reference voltage.
 13. Detector accordingto claim 12, wherein the regulator is constituted by a comparator (A₃)of which one of the inputs is connected to a reference voltage sourceand the other input to a point of the circuit situated downstream of thegenerator.